[Zope-CMF] 'portal_membership.getMemberById' became inaccessible

[Kari-Hans Kommonen]

we are running a cmf site and suddenly noticed that anywhere we use

  member = context.portal_membership.getMemberById( x )

we are redirected to login apparently because of insufficient rights, 
regardless of who the current user is. Also, we usually call it in a 
script that has a 'Manager' proxy role, so it should always have 
adequate access rights.

We just noticed this change in behaviour, and we have not knowingly 
done anything to the site. Restarts do not change the situation, and 
the member data seems valid and is still available through other, 
nonprivileged calls, and through external code.

What could be the reason? How to debug this?

TIA, khk